Certain materials, for example silicon, having properties that are particularly interesting in the fields mentioned above, can be obtained in large quantities and with excellent quality using ingot pulling techniques that are well known to the skilled person.
The ingots obtained are then readily sliced into wafers which, after a large number of processing steps, become substrates that are used to produce integrated circuits, for example.
Other materials such as gallium arsenide, gallium nitride, indium phosphide, germanium, or silicon carbide, are also of interest. However, not all of those materials can be obtained by pulling ingots of high crystalline quality. Further, it is not always possible to fabricate substrates from such ingots because the costs are too high or the method is too difficult to implement. Thus, for example, existing methods are not suitable for fabricating gallium nitride (GaN) ingots on an industrial scale.
The document “Bulk and Homoepitaxial GaN Growth and Characterization”, Porowski-S, Journal of Crystal Growth, vol 189-190, June 1998, pp. 153-158, describes a method of growing a monocrystalline GaN ingot in the liquid phase at a pressure of 12 to 20 kbars (12 to 20×108 Pascals (Pa)) and at a temperature in the range 1400° C. to 1700° C. However, those conditions are difficult to implement under mass production conditions. Further, they can only produce crystals with a maximum diameter of 18 millimeters (mm).
Other research teams have also worked on a method of growing an ingot in the liquid phase at reduced pressure (less than 2 bars (2×105 Pa)) and at a temperature of 1000° C. The diameter of the crystals obtained is larger, close to 50 mm, but the crystalline quality obtained is not as good as with the above-mentioned technique.
Finally, the document “Growth and Characterization of GaN Single Crystals”, Balka et al, Journal of Crystal Growth, vol 208, January 2000, p 100-106, discloses the growth of monocrystalline GaN by sublimation. The fabrication conditions employed are a pressure of less than 1 bar (105 Pa) and a temperature of 1000° C. to 1200° C. The crystalline quality obtained is very high, but the crystal size is 3 mm, which is clearly insufficient for the envisaged applications. Thus, the market currently offers no monocrystalline gallium nitride in the bulk form, of high quality, of a sufficient diameter, and at a reasonable price.
The prior art discloses a number of attempts to fabricate substrates by epitaxy or heteroepitaxy in order to overcome the problem of obtaining certain specific materials.
Epitaxy can combine materials with different natures in the form of thin films and combine their properties to produce components, for example high electron mobility transistors (HEMTs), diodes, or lasers.
Heteroepitaxy involves depositing the desired material onto a support of different crystallographic nature by epitaxy, and then eliminating the support, if possible and necessary for the remainder of the process. The main drawback of that technique is that the material constituting the support and that deposited by epitaxy generally have different lattice parameters and thermal expansion coefficients.
The differences in lattice parameters between the support and the epitaxial layer create a large number of crystalline defects in the epitaxially grown material, such as dislocations or stacking defects, for example.
Added to that is the fact that epitaxial growth is generally carried out at high temperatures, above 600° C. and possibly up to 1000° C. to 1100° C., for example, when growing gallium nitride epitaxially by metal organic chemical vapor deposition (MOCVD). For that reason, as the structure which is formed cools to ambient temperature, the epitaxial layer obtained develops a number of residual stresses and strains connected with differences in thermal expansion between it and its support.
To overcome that drawback, the material selected as the support preferably has crystalline structure and thermal expansion coefficient very close to those of the materials which are to be grown epitaxially. As an example, gallium and indium arsenide or gallium and aluminum arsenide can be grown epitaxially on a gallium arsenide support with crystallographic quality that is sufficient to produce components.
However, other materials do not always have a compatible support that is in the form of a substrate. This is particularly the case with materials such as gallium nitride or cubic silicon carbide.
Until now, components having one of those two materials as the active layer have been grown by heteroepitaxy.
Thus, for gallium nitride, light-emitting diodes (LEDs) and lasers emitting in the blue, violet and ultraviolet as well as high frequency power components have been produced using sapphire, hexagonal silicon carbide, or silicon as the support.
For silicon carbide, which is unavailable in the form of a substrate in its cubic crystalline structure, micro-electromechanical components (MEMS) or power transistors have been produced by depositing silicon carbide onto a silicon substrate by epitaxy.
However, in order to further improve the quality of the components obtained, it would be desirable to fabricate bulk gallium nitride or cubic silicon carbide substrates of the same nature as the epitaxially grown layer deposited over it.
Attempts made in the past have resulted in products with a certain number of disadvantages. As an example, one intermediate route consists in using a technique termed “high growth rate epitaxy” to produce an epitaxially grown film that is as thick as the substrate supporting it. That support substrate is then eliminated and only the thick epitaxially grown film is retained, which in turn becomes a substrate for conventional epitaxy. Methods of that type exist for producing gallium nitride and silicon carbide, but the quality of the substrates obtained is generally mediocre because of the influence of the original support substrate of different crystallographic nature.
Thus, typically, large residual stresses are observed in the case of the epitaxial growth of cubic silicon carbide on silicon. Such stresses generally result in very substantial curvature of the epitaxially grown silicon carbide once the original silicon support substrate has been removed. That curvature renders it unusable for all subsequent forming steps.
Similarly, for the gallium nitride obtained, the influence of the support substrate is illustrated by the appearance of a very large number of dislocations and by cracking of the epitaxially grown film as its temperature falls, in particular when that epitaxially grown film exceeds a certain critical thickness.
U.S. Pat. No. 6,146,457 describes a further method that makes use of the stresses that appear as the temperature falls following epitaxy as the driving force for detaching an original support substrate from a thick epitaxially grown layer. That result is obtained by inserting a layer termed a “weak” layer between the support substrate and the thick epitaxially grown layer such that when the system stresses increase (as happens during cooling), the weak layer ruptures naturally and thus ensures controlled detachment of the support from the thick epitaxial layer. However, that detachment technique is difficult to control with a large specimen. Further, it involves inserting a particular layer as epitaxial growth commences or during epitaxial growth, which may be deleterious to the crystallographic quality of that epitaxially grown layer.
Further, the document “Physical Properties of Bulk GaN Crystals Grown by HVPE”, Melnik et al, MRS Internet Journal of Nitride Semiconductor Research, vol 2, art 39, describes a method of growing gallium nitride (GaN) monocrystals by HVPE on a monocrystalline silicon carbide (SiC) substrate, and removing that substrate using a reactive ionic etching (RIE) technique. However, that SiC substrate takes a long time to remove since it is highly inert chemically.
Finally, the document “Large Free-standing GaN Substrates by Hydride Vapor Phase Epitaxy and Laser-Induced Lift-Off”, Kelly et al, Jpn J Appl Phys, vol 38, 1999, describes a method of growing GaN by hydride vapor phase epitaxy (HVPE) on a sapphire substrate, then removing that substrate by laser-induced lift-off. That technique is based on using a laser of a wavelength that is absorbed only by gallium nitride and not by sapphire. Scanning the resulting structure with the laser ensures that the two materials become detached by local modification of the properties of the GaN after passage of the laser.
This lift-off technique, however, is difficult to implement when treating large areas, since laser beam scanning takes a long time.
It is also possible to remove the sapphire substrate by mechanical polishing, but that method is also lengthy and furthermore, it runs the risk of breaking the gallium nitride layer when lifting off the substrate, by releasing the existing stresses.
Thus, there is a need for improved methods of making an epitaxially grown layer of high crystallographic quality that can be readily detached from its epitaxial support, especially for materials that have previously only been obtainable by heteroepitaxy. The present invention now satisfies this need.